first American-built rigid airship, the , flew in 1923. The Shenandoah was the first to use helium, which was in such short supply that the one airship contained most of the world's reserves. The US Navy explored the idea of using airships as airborne aircraft carriers. Whereas the British had experimented with an aircraft "trapeze" on the R33 many years before, the Americans built hangars into two new airships and even designed specialist airplanes for them. The and were the world's largest airships at the time, with each carrying four F9C Sparrowhawk fighters in its hangar. Although successful, the idea was not taken further. By the time the Navy started to develop a sound doctrine for using these airships, both had been lost in accidents. More significantly, the seaplane had become more mature and was considered a better investment. The Empire State Building, then the tallest building in the world, was completed in 1931 with a dirigible mast, in anticipation of passenger airship service. The most famous airships today are the passenger-carrying rigid airships made by the German Zeppelin company, especially the Graf Zeppelin of 1928 and the Hindenburg of 1936. The Graf Zeppelin was intended to stimulate interest in passenger airships, and was the largest airship that could be built in the company's existing shed. Its engines ran on blau gas, similar to propane, which was stored in large gas bags below the hydrogen cells. Since its density was similar to that of air, it avoided any weight change as fuel was used, and thus the need to vent hydrogen. The Graf Zeppelin became the first aircraft to fly all the way around the world. Airship operations suffered a series of highly publicised fatal accidents, notably to the British R101 in 1930 and the German Hindenburg in 1937. Following the Hindenburg disaster, the age of the great airships was effectively over. Aeronautical advances During the late 1920s and early 1930s the available power from aero engines increased significantly, making possible the adoption of the fast cantilever-wing monoplane, originally pioneered as far back as late 1915. The ability to handle the high mechanical stresses imposed by this advanced form of airframe design philosophy suited the all-metal aircraft construction techniques pioneered by some earlier designers, and the increasing availability of high strength-to-weight aluminum alloys — first used by Hugo Junkers in 1916-17 as duralumin for his all-metal airframe designs — made